Method of and apparatus for the recovery of heat and chemicals from hot dust laden gas



METHOD OF AND APPARATUS FOR THE RECOVERY OF HEAT CHEMICALS FROM HOT DUSTLADEN GAS 1959 AND Original Filed Nov. 12,

Fur macs HoT DUST LADEN GAs l N CLEAN COOLED GAS OUT Low DENSITY LIQUORIN LIQUOR OUT H. P. MARKANT 2 Sheets-Sheet 1- INVENTOR.

Henry R Markanr ATTORNEY FIG.3

Oct. 19, 1965 H. P. MARKANT 3,212,235

METHOD OF AND APPARATUS FOR THE RECOVERY OF HEAT AND CHEMICALS FROM HOTDUST LADEN GAS 2 Sheets-Sheet 2 Original Filed Nov. 12, 1959 t c g L S ID 2 5 INVENTOR.

Henry R Markanr BY ATTORNEY United States Patent METHOD OF AND APPARATUSFOR THE RECOV- ERY OF HEAT AND CHEMICALS FROM HOT DUST LADEN GAS HenryP. Marlrant, Alliance, Ohio, assignor to The Babcoek & Wilcox Company,New York, N.Y., a corporation of New Jersey Continuation of applicationSer. No. 852,501, Nov. 12,

1959. This application Feb. 14, 1962, Ser. No. 174,657 Claims. (Cl.5548) This application is a continuation of my prior application SerialNo. 852,501 filed November 12, 1959, now abandoned.

This invention relates to a method of and apparatus for the recovery ofheat and solids from a hot gas laden with entrained dust particles and/or chemical fume, and more particularly to a method of and apparatus forincreasing dust removal efficiency without increasing the powerrequirements for such recovery.

In many industrial processes the gases produced contain chemicals in theform of dust or fume which it is desirable to separate out to recovermonetary values and/ or to eliminate a dust nuisance. When the gases inaddition contain heat values, it may also be of economic importance toreclaim at least some of the heat in the gases. The desirability of bothheat and chemical recovery is exemplified in the pulp and paper industryapplications where the effluent gases from the incineration of the pulpresidual liquor from, for example, the sulphate process, containchemical fume and heat values.

It is known that the dust or fume in gases discharged from a smeltingfurnace used in the chemical recovery of residual liquor, may be removedfrom the gases by use of a venturi scrubber-cyclone separator unit suchas shown in US. Patent 2,879,838. It is also known that the fume may beremoved from the hot gases by intimate contact between the hot gases anda sprayed liquid such as water or brine. The intimate contact isattained by the acceleration of the gases to a high velocity, such as175 ft. per second, where the high velocity gases atomize the relativelycoarse spray of the introduced liquid. When the gases contacted by theliquid are at a relatively high temperature, considerable quantities ofthe spray liquid will be evaporated. Under these circumstances, theliquid droplets atomized in the venturi scrubber will tend to wet andthe dust particles agglomerate and absorb the fume in the gases.Thereafter, the agglomerated particles may be separated from the gasesin the cyclone separator portion of the unit. It is also known that thefume may be removed from the effluent gases by the use of residualliquor as the contacting medium. Thus, under these circumstances, notonly is the fume removed from the entraining gases but the residualliquor is also concentrated by the evaporation of moisture therefrom sothat the liquor may thereafter be used directly in the incineratingfurnace. However, the use of residual liquor as a spraying mediumnecessitates increasing the flow velocity and pressure drop of the gasespassing through the throat of the venturi scrubber. The increasedpressure drop is necessary due to the viscosities of the liquor wherehigher velocities are needed to suitably atomize the sprayed liquor. Asa result of the use of liquor, the pressure drop of the gases passingthrough the throat approaches the economic limits of fans when highefliciency dust separation is desired.

In accordance with this invention, I have found that the advantageousconcentration of the residual liquor and an efficient recovery of thedust in the effluent gases can be attained by the use of residual liquorof different concentrations introduced at spaced positions along thelength of a venturi scrubber. With my invention, I can in- "Ice creasethe dust collecting efliciency of a venturi scrubber without increasingthe pressure drop of the gases flowing through the scrubber as comparedto present practice. Conversely, I can reduce the pressure drop of thegases passing through the scrubber while maintaining the dust collectingefliciency of the unit.

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which I have illustrated and described a preferred embodimentof the invention.

Of the drawings:

FIG. 1 is a schematic showing of a venturi scrubbercyclone separatorconstructed and arranged in accordance with this invention;

FIG. 2 is an enlarged View of a portion of the venturi scrubber shown inFIG. 1;

FIG. 3 is a side elevation of the venturi scrubber throat portion shownin FIG. 2; and

FIG. 4 is an enlarged sectional view of the throat por tion of thescrubber shown in FIG. 2, taken along line 4-4.

As shown in FIG. 1, a venturi scrubber-cyclone separator unit of thegeneral type shown in US. Patent 2,879,838 is diagrammaticallyillustrated. The hot gases received from, for example, the incineratingfurnace of a residual liquor recovery unit enter the venturi scrubberthrough a duct 10. The gases pass through a converging inlet portion 11wherein the gases are accelerated prior to entering the throat portion12 of the scrubber. After leaving the throat portion 12, the gases, withentrained liquids and solids, pass through a diverging portion 13, aturning duct portion 14 and tangentially enter the cyclone separator 15.Within the cyclone 15, the gases whirl in an upward direction to passthrough a central gas outlet duct 16 in the upper end of the cyclone.The whirling action of the gases in passing through the cyclone causes acentrifugal separation of the entrained solid and liquid particles, withthose particles striking the wall of the cyclone and discharging bygravity to an inverted frusto-conical false bottom 17 in the cyclone.The false bottom is provided with a central outlet 18 which dischargesinto a sump 20 formed between the frusto-conica1 false bottom and thelower inverted frusto-conica1 plate 21 of the cyclone 15.

Since the particles separated in the cyclone may have a tendency toaccumulate on the interior surfaces of the cyclone, wall washing meansare provided to remove the particles. As shown, residual liquor isdelivered from a pump 22 through pipe 23 and branch pipe 24 to a seriesof nozzles 25 positioned immediately adjacent the top cover plate 26 ofthe cyclone separator 15. A frustoconical shield or skirt 27 encirclesthe duct 16 forming the gas outlet from the separator with its loweredge portioninwardly spaced from the interior surface of the walldefining the cyclone. The nozzles 25 discharge the wall washing liquorinto the annular space above the frustoconical skirt 27 causing theliquor to move downwardly along the wall of the cyclone, with themixture discharging to the sump 20 through the central opening 18 of thefalse bottom 17.

As shown in FIG. 1, the residual liquor delivered to the venturiscrubber is passed to the nozzle assemblies 28 arranged .in oppositesides of the converging section 11 through pipe 23 from the dischargeside of the pump 22. The pump in turn receives liquor from the sump 20in the cyclone separator. A second branch pipe 36 leads from the pipe 23to the point of use, which in the case of a pulp mill recovery systemwould be the spray nozzle supplying the incinerating furnace of the heatand chemical recovery unit (not shown). The liquor withdrawn from thesump in the bottom of the cyclone separator and delivered to the spraynozzle of an incinerating furnace will ordinarily have a concentrationof the order of 60 to 65% solids, for self-sustaining combustion of theliquor .in the incinerating furnace.

When the washing liquor delivered to the throat has a solids content ofthe order of 60%, the density of the liquor is of the order of 82 lbs.per cubic foot and the viscosity will be about 600 Saybolt Universalseconds at 200 F. Under these conditions with nozzle assemblies 32 ofthe type illustrated in FIG. 4, satisfactory atomization of the liquorwill occur with a gas velocity of 175 ft. per second (or greater)through the throat 12, and 85% to 90% of the dust or fume in the gasdelivered to the scrubher-separator unit will be removed with a gas flowpressure drop through the unit of the order of 33 to 35 inches of waterpressure.

Substantially the same dust removal efficiency can be obtained whenpassing the gases through the unit using a spray liquid which has aviscosity equal to that of water. For this condition the gas flowvelocity may be greatly reduced with a corresponding reduction inpressure drop to the order of inches of water pressure. However, it iseconomically desirable to use residual and/ or makeup liquor as thespray medium since the heat in the gas will then be utilized toconcentrate the liquor and thus save the cost of steam necessary toconcentrate the liquor to a solids content usable in the incineratingfurnace.

The efficiency of dust or fume removal in the scrubberseparator unit isdependent upon the intimacy of contact between the liquid droplets andthe dust or fume particles, and upon the fineness of the liquid dropletsavailable for the gas-liquor contact in the venturi. Since high gasvelocities are necessary to atomize the liquid introduced into theventuri, it should be noted that increasing the viscosity of the liquidproduces a corresponding increase in the gas pressure drop.

Make-up liquor supplied to the scrubber-separator combination isobtained, in the case of the pulp mill installation, from a multipleeffect evaporator where the pulp residual liquor is concentrated to asolids content between 40% and 50%. The amount of liquor used as make-upfrom the multiple effect evaporators will be dependent upon the rate ofliquor withdrawal through the pipe to the incinerating furnace. Inaccordance with the present invention, the makeup liquor at, forexample, a solids concentration of 45% is delivered through a pipe 31 tothe nozzle assemblies 32 positioned in the venturi throat downstream, ina gas flow sense, of the nozzle assemblies 28. It will be understoodthat the higher the viscosity, or the density, of the liquor deliveredto the venturi throat, the greater the velocity of gas flow necessarythrough the throat 12 to attain the desired subdivision of the liquorand wetting of the fine dust particles in the gas.

With the makeup liquor received from the multiple effect evaporatorshaving a solids concentration of 45 for example, the density of thisliquor will be of the order of 75 lbs. per cubic foot at 200 F. Whilethe densities of the 60% solids concentration and 45% solidsconcentration liquors are not greatly different, i.e., 82 lbs. per cubicfoot and 75 lbs. per cubic foot respectively, the viscosity of the twoliquors will be of the order of 600 Saybolt Universal seconds and 50Saybolt Universal seconds, respectively.

Since the effectiveness of gas-liquid contact in the venturi throat isdependent in the main upon the viscosity of the liquid introducedthereto, the lower density makeup liquor may be effectively used in thepresent invention to accomplish an efficient removal of fume or dustfrom the gas while at the same time effectively concentrating the liquorby evaporation to a usable value for subsequent use in the incineratingfurnace.

It will be understood that the rate of liquor How to the venturiscrubber will be as much as 10 or more times the rate of liquor flow tothe incinerating furnace, in order to attain the necessary degree ofdust removal and liquor evaporation for the purpose described. Forexample, the liquor to gas ratio in the scrubber may be of the order of20 gal./ 1,000 cu. ft., whereas the rate of makeup liquor flow to thenozzles 32 may be 2 gal/1,000 cu. ft. While the makeup liquor, i.e., thelow viscosity liquor, may be introduced into the scrubber upstream ofthe nozzle position of the recirculated high viscosity liquor, thepreferred arrangement is as shown in the drawings where the nozzleassemblies 32 are downstream of the nozzle assemblies 28. The latterarrangement is preferred to protect the low flow rate nozzles from theplugging effects occasioned by contact with the high temperatureentering gases.

The enlarged views of the venturi shown in FIGS. 2 and 3 illustrate apreferred construction. As shown in FIG. 2, the walls 34 and 35 of theinlet portion 11 leading to the throat 12 converge to reduce thedimension of the throat to a value which is but a small fraction of thecorresponding dimension of the attached duct 10. As shown in FIG. 3, theside view of the throat illustrates the substantial uniformity in thewidth of the duct 10, the inlet and outlet portions 11 and 13respectively, and the venturi throat 12. The nozzle assemblies 28 forintroducing the higher density liquor are supplied through headers 36,which receive the liquor from pipe 23. Each header 36 is connected withstub headers 37 by pipes 38 each of which is provided with a flexibleconnection 40 and a shut-off valve 41. Each of the stub headers 37 isprovided with a plurality of equally spaced orifices and a matchingnozzle for the discharge of liquor into the scrubber. To assist in theatomization of the high viscosity recirculated liquor, each of theliquor streams in an individual nozzle is provided with an intersectingjet of a highly expansible fluid, such as steam. As shown in FIGS. 3 and4, the steam passes from a supply pipe 42 through a valved pipe 43 to asteam header 44 attached to an opening to each nozzle.

As shown in FIG. 4, the stub header manifold 37 is mounted directly onthe exterior side of the plate 35 (or 34) by brackets 45 with eachorifice 46 opening to a hole 46 drilled and threaded into a bar 47 whichis secured to the stub header and bears on the exterior edge portions ofan opening A formed in the plate 35. An orifice plug 43 is threaded intothe bar 47 with the opening 46 coaxial with the orifice 50 of the plug48. The bar 47 is drilled to connect the header 44 with the hole 46 andthe plug is also drilled to provide a passageway 51 opening to theorifice 50 so that steam may be admitted into contact with the liquorcausing its discharge as a finely atomized mist into the venturi. Whensteam is used for atomizing purposes, the steam quantity may beapproximately 2% by weight of the high viscosity liquor delivered to thescrubber. Good atomization can be obtained by using steam at a pressurein the order of 15 pounds per square inch greater than the liquorpressure in the manifold 37.

The low viscosity liquor passing through the pipe 31 is delivered tomanifolds 52, each mounted by brackets 53 on a side wall of the throat12 downstream, in a gas flow sense, of the nozzle assemblies 28. Themanifolds 52 are each provided with a plurality of equally spacedorifice openings 54 in a side thereof, with the row of openings enclosedby bars 55 and 56 which are attached to the manifold and bear upon theedge portions of an opening in the wall of the throat 12. A splash plate57 is attached to the manifold immediately below the orifice openings 54and extends in an upwardly inclined direction to the throat in relationto the gas fiow path effecting the discharge of fiat sheets of liquorfrom opposite sides of the throat. Preferably, the splash plate ispositioned so its forward edge does not project into the path of the gasmoving through the throat.

In the structural arrangement shown in FIG. 4, each of the nozzles 28and 32 are arranged so that they may be removed for cleaning orreplacement. In the usual installation, each set of nozzles is arrangedin a row with the spacing of the nozzles in the assemblies 28 of theorder 2 /2 while the nozzles in the assemblies 32 are arranged on 4"spacing, so as to obtain adequate and complete distribution of theliquor across the gas flow passageway. Other nozzle spacings may be usedon some installations without adverse eiTect on the operation of thescrubber, provided all portions of the gas stream passing through theventuri are contacted by both high and low viscosity liquor.

While in accordance with the provisions of the statutes I haveillustrated and described herein the best form and mode of operation ofthe invention now known to me, those skilled in the art will understandthe changes may be made in the form of the apparatus disclosed withoutdeparting from the spirit of the invention covered by my claims, andthat certain features of my invention may sometimes be used to advantagewithout a corresponding use of other features.

What is claimed is:

1. Apparatus for concentrating residual pulp liquor and for scrubbingchemical dust and fume from hot flue gases discharged from a furnacewherein the concentrated liquor is burned, said apparatus comprising aventuri scrubber, a gas and liquid separating means, and means forpassing a flue gas from a furnace serially through said scrubber andsaid separating means, said scrubber including a vertically disposedunobstructed conduit having an inlet portion of gradually diminishingcross-sectional flow area for accelerating said gases, a throat portionof constant cross-sectional flow area, a discharge portion of graduallyincreasing flow area arranged to effect a substantial regain of pressureof said gases, and spray means for introducing concentrated pulp liquorinto the accelerated hot gas stream within said conduit, said separatingmeans being arranged for the separation of liquor from said gas streamand collection of a concentrated liquor, said spray means comprising afirst set of nozzles connected to said inlet portion and constructed andarranged to spray said concentrated liquor substantially onlytransversely into the accelerated gas stream in the vicinity of saidthroat portion prior to its entering the discharge portion of saidconduit, means including pump means for supplying the concentratedliquor from said separating means to said first set of nozzles, meansfor introducing a highly expansible fluid through said first set ofnozzles along with said concentrated liquor to produce a finely dividedspray of said concentrated liquor above said throat portion, a source ofresidual pulp liquor having a viscosity substantially less than that ofsaid concentrated liquor, and means including a second set of nozzlesconnected to said throat and said source arranged for introducing all ofsaid residual liquor from said source transversely into the acceleratedgas at the throat portion of said conduit.

2. Apparatus according to claim 1 wherein means interconnect saidfurnace and said separating means for discharging a controlled quantityof said concentrated liquor from the separating means to the furnace forburning said liquor.

3. The method of concentrating residual pulp liquor and recoveringchemical dust and fume from hot flue gases discharged from a furnacewherein concentrated liquor is burned and from which combustion gasespass through a venturi shaped evaporator-scrubber having converginginlet and diverging outlet portions and an intermediate throat portionand thence to a gas and liquid separating means wherein concentratedliquor is separated and collected, which comprises passing the fluegases into the converging portion, atomizing concentrated liquor fromsaid gas and liquid separating means with a highly expansible gas,introducing the atomized liquor into the converging portion of saidevaporator-scrubber at a rate substantially greater than the rate ofcombustion of said concentrated liquor in the furnace to cause itsdischarge as a finely atomized mist into intimate contact with the fiuegases, and spraying a relatively low concentration make-up liquortransversely of the flue gases in the throat portion in contacttherewith and at a rate substantially compensating for the rate ofconcentrated liquor combustion.

4. In a method according to claim 3, wherein the higher concentratedliquor is recirculated a rate of the order of 10 times the normal rateof delivery of the relatively low concentration make-up liquor.

5. In a method according to claim 3 wherein the concentrated liquor hasa viscosity of the order 600 Saybolt Universal seconds and the viscosityof the relatively low concentration make-up liquor is of the order of 50Saybolt Universal seconds.

Refereuces Cited by the Examiner UNITED STATES PATENTS 2,590,905 4/52Tomlinson et a1 159-4 2,604,185 7/52 Johnstone et al 55237 2,621,75412/52 Doyle.

2,797,904 7/57 Voorheis 261-118 2,829,710 4/58 Paasch 1594 2,871,9732/59 Roujob 261-118 X 2,879,838 3/59 Flynt et a1. 159-47 2,895,335 7/59Kraftson et a1. 73-4215 2,935,375 5/60 Boucher 55257 X 2,987,921 6/61Kraftson 73-421.5 3,009,687 11/61 Hendriks 261-118 X FOREIGN PATENTS809,378 2/59 Great Britain.

REUBEN FRIEDMAN, Primary Examiner. HERBERT L. MARTIN, Examiner.

1. APPARATUS FOR CONCENTRATING RESIDULA PULP LIQUOR AND FOR SCRUBBINGCHEMICAL DUST AND FUME FROM HOT FLUE GASES DISCHARGED FROMA FURNACEWHERIN THE CONCENTRATED LIQUOR IS BRNED, SAID APPARATUS COMPRISING AVENTURI SCRUBBER, A GAS AND LIQUID SEPARATING MEANS, AND MEANS FORPASSING A FLUE GAS FROM A FURNACE SERIALLY THROUGH SAID SCRUBBER ANDSAID SEPARATING MEANS, SAID SCRUBBER INCLUDING A VERTICALLY DISPOSEDUNOBSTRUCTED CONDUIT HAVING AN INLET PORTION OF GRADUALLY DIMINISHINGCROSS-SECTIONAL FLOW AREA FOR ACCELERATING SAID GASES, A THROAT PORTIONOF CONSTANT CROSS-SECTIONAL FLOW AREA, A DISCHARGE PORTION OF GRADUALLYINCREASING FLOW AREA ARRANGED TO EFFECT A SUBSTANTIAL REGAIN OF PRESSUREOF SAID GASES, AND SPRAY MEANS FOR INTRODUCING CONCENTRATED PULP LIQUORINTO THE ACCELERATED HOT GAS STREAM WITHIN SAID CONDUIT, SAID SEPARATINGMEANS BEING ARRANGED FOR THE SEPARATION OF LIQUOR FROM SAID GAS STREAMAND COLLECTION OF A CONCENTRATED LIQUOR, SAID SPRAY MEANS COMPRISING AFIRST SET OF NOZZLES CONNECTED TO SAID INLET PORTION AND CONSTRUCETEDAND ARRANGED TO SPRAY SAID CONCENTRATED LIQUOR SUBSTANTIALLY ONLYTRANSVERSELY INTO THE ACCELERATED GAS STREAM IN THE VICINITY OF SAIDTHROAT PORTION PRIOR TO ITS ENTERING THE DISCHARGE PORTION OF SAIDCONDUIT, MEANS INCLUDING PUMP MEANS FOR SUPPLYING THE CONCENTRATEDLIQUOR FROM SAID SEPARATING MEANS TO SAID FIRST SET OF NOZZLES, MEANSFOR INTRODUCING A HIGHLY EXPANSIBLE FLUID THROUGH SAID FIRST SET OFNOZZLES ALONG WITH SAID CONCENTRATED LIQUOR TO PRODUCE A FINELY DIVIDEDSPRAY OF SAID CONCENTRATED LIQUOR ABOVE SAID THROAT PORTION, A SOURCE OFRESIDUAL PULP LIQUOR HAVING A VISCOSITY SUBSTANTIALLY LESS THAN THAT OFSAID CONCENTRATED LIQUOR, AND MEANS INCLUDING A SECOND SET OF NOZZLESCONNECTED TO SAID THROAT AND SAID SOURCE ARRANGED FOR INTRODUCING ALL OFSAID RESIDUAL LIQUOR FROM SAID SOURCE TRANSVERSELY INTO THE ACCELERATEDGAS AT THE THROAT PORTION OF SAID CONDUIT.